Liquid transfer development



y 7, 1966 v. s. MIHAJLOV 3,251,688

LIQUID TRANSFER DEVELOPMENT Filed July 2, 1962 M K M W WNW W32 F/aa INVENTOR. VSEVOLOD S. M lHAJLOV lika 3 Q Q ATTORNEY tion is described in detail below.

United States Patent 3,251,688 LIQUID TRANSFER DEVELOPMENT Vsevolod S. Mihajlov, Rochester, N.Y., assignor to Xerox Corporation, Rochester, N.Y., a corporation of New York Filed July 2, 1962, Ser. No. 206,921

11 Claims. (Cl. 961) This invention relates to xerography, and in particular to the production of copies by liquid transfer-development.

The production of copies of drawings, documents, or printed matter by xerography is now well known. According to one method a latent electrostatic image is formed on a photoconductive insulating layer and then rendered visible by the selective deposition of an electrostatically attracta'ble dyestuif. This, visible image may then be transferred to a suitable transfer material such as paper, plastic, or the like, thus freeing the plate for further use. If desired, the transferred image may be permanently fixed to the transfer materialby any suitable method such as heat or vapor fusing, or the fixing step may be obviated by using a self-fixing 'dyestuff. Both dry and liquid development systems for depositing the dyestuff are known to the art and have been described in various patents and other publications.

Especially with respect to liquid systems, however, effective transferis often difficult or inconvenient. Special apparatus is typically necessary and even with such apparatus the image tends to hold tenaciously to the plate surface. The maintenance of image stability during the transfer process may present additional problems by limiting the usable materials and manipulation speeds. Nevertheless, methods utilizing liquid developers in=systems utilizing reusable plates are of interest because the generally high degree of image resolution attainable and for their suitability for the reproduction of solid areas.

Now in accordance with the present invention, there is provided a method of reusable plate xerography for making copies directly on tranfer material using liquid developers. As set forth in the following description, the present invention eliminates the customarily separate transfer step and instead teaches a novel system of transfer-development from a plate pre-coated with liquid developer. Thus, there is provided a practical syst m for realizing the advantages of liquid development in xerography. Various additional purposes and objectives of the present invention will be made apparent by the following specification and claims. comprising the instant technique are illustrated by the accompanying drawings in which:

FIG. 1 shows one method of coating a xerographic plate with liquid developer;

FIG. 2 illustrates the application of electrostatic charge;

FIG. 3 illustrates exposure of the charged plate to a pattern of light and shadow;

FIG. 4 illustrates transfer-development onto image receiving material;

FIG. 5 illustrates simultaneous transfer-development and exposure; and

FIG. 6' illustrates a liquification step necessary in certain embodiments of the present invention.

FIG. 1 illustrates the immersion method of coating a Xerographic plate 10 with liquid developer 11. The plate 10 is suspended in a tank 12 containing a liquid developer 11 by a string 13 which is wrapped around the shaft of a slow speed motor 14 such as a clock motor. The liquid developer 11 employed in the present inven- It preferably comprises finely divided electrically charged dyestuff, herein termed toner, dispersed in a relatively non-conductive liquid carrier.

The several steps 3,251,688 Patented May 17, 1966 When motor 14 is energized, plate 10 is slowly withdrawn from tank 12 and a thin coating is formed on the surface of plate 10. Of course, other suitable coating techniques may alternatively be employed such as pouring electrophoretic coating, spin coating, and vacuum coating.

Xerographic plate 10, more clearly shown in F IG.-2, comprises a photoconductive insulating layer 15'overlying conductive backing 16 which may or may not be transparent depending on the particular embodiment of this invention. Where the plate is desirably transparent, Nesa glass has been found suitable for conductive backing 16. This material is believed to be glass to which tin oxide has been applied so that at least one surface is electrically conductive.

As shown in FIG. 2, a uniform electrostatic charge of the same polarity as that of the toner is applied to the plate by corona discharge electrode 18. A suitable corona discharge electrode may comprise one or more fine conductive strands 19 maintained at a corona discharge potential by high voltage source 21. Conductive strands 19 are positioned within a backing shield 22 and are maintained at a desired corona discharge potential such as, for example, a potential in the order of several thousand volts and may be controlled to deposit on the surface electrical charge of the desired polarity. As illustrated in FIG. 2, corona discharge electrode 18 is being moved across the plate surface from left to right to deposit electrical charge on the liquid developer layer previously deposited on the plate surface.

FIG. 3 illustrates exposure of the charged X rographic plate to a pattern of light and shadow. Photographic enlarger 24 is shown projecting a light pattern upon xerographic plate 10. It will be noted that, if conductive backing 16 is transparent in a particular embodiment, exposure may be effected through the opposite face of plate 10 from that shown in FIG. 3. Other methods or apparatus for exposure may be employed such as exposure in a camera or the like. 4

After completing the foregoing steps, a copy may be made -on suitable transfer material by the transferdevelopment procedure illustrated in FIG. 4. Image receiving Web 26 is brought into light pressure contact with the layer of liquid developer by moving roller 27 across the web in the direction indicated by the arrow. This results in the formation of toner image 28 corresponding to the non-light-struck areas of plate 10 on image receiving web 26.

Roller 27 desirably comprises an electrically conductive material such as conductive rubber,'although other materials including non-conductive materials may be used. When a conductor is employed, print quality is enhanced by applying an electrical potential to the roller. Relatively small quantities of toner may adhere to the image receiving material in non-image areas thereby causing the appearance of background in the prints if the roller is allowed to electrically float. This undesired phenomenon can be appreciably reduced by applying an electrical potential of the same polarity as that of the toner to roller 27. Electrical potentials between about ground and about the potential of the discharge areas of the surface bearing the charge pattern have been found generally suitable. comparatively light copies having a washed out appearance have been found to result when a potential of greater magnitude is applied to the roller. On the other hand, substantial background in non-image areas is noted when an electrical potential of opposite polarity of that of the toner is applied to roller 27.

FIG. 5 shows an embodiment of the present invention found suitable for carrying out the transfer-development procedure simultaneously with the exposure step. In this embodiment, Xerographic plate 10 comprises a photoconductive layer 15 overlying a transparent backing 32 such as glass or plastic. To impart the desired electrical characteristics, at least one side of transparent backing =32 may have a conductive coating 33 such as tin oxide. Nesa glass, mentioned earlier, has been found suitable for use as the transparent conductive backing member.

Xerographic plate '10 is first prepared by coating and charging as described in connection with FIGS. 1 and 2. Then, as shown in FIG. 5, image receiving web 34 is brought into mild pressure contact with liquid developer 11. While these members are so positioned, xerographic plate is exposed to a pattern of light and shadow to be reproduced. The vertical arrows represent light rays, and are shown as projected toward the transparent backing side of the plate. This manipulation results in the transfer of toner particles to the image receiving web 34 in the dark areas to form toner image 28. Toner image 28 may then be removed on image receiving web 34 by separating it from xerographic plate 10*.

As described'in connection with FIG. 1 and FIG. 2, the xerographic plate is coated with liquid developer before it is charged. However, in carrying out the present invention these operations may be per-formed in either order. Thus, if desired, the plate may be charged before it is coated with liquid developer.

Reversal development, or toner deposition conforming to discharged areas of the plate, may be accomplished by using a suitable toner which is electrically charged to a polarity opposite to that of the electrostatic charge applied to the plate. For such applications of the instant invention it is desriable that an electrical potential of opposite polarity to that of the toner be applied to the pressure roller. Thus, for example, an appropriate combination for reversal developement would comprise using a positively charged toner, applying a negative electrostatic charge to the plate, and biasing the pressure roller with a negative electrical potential.

Although the mechanism of transfer-development is not fully understood, the foregoing description in connection with FIG. 4 suggests that toner particles ade deposited on the image receiving web in relation to electrostatic fields of force. The charged toner particles may be repelled toward the Web by the electrical charge remaining on the plate after the exposure step corresponding to the dark areas of the pattern of light and shadow. Transfer is not effected in the light areas, since these are substantially discharged as a result of the exposure step.

However, it has also been observed that after exposure, developer in the dark areas is slightly raised with respect to developer in the light areas. Results obtained by carefully regulating the positioning of the web during the transfer-developement step suggests that toner may be seelctively transferred by bringing the web into contact with only these raised portions. Possibly aided by the electrical forces of the remaining charge, toner sticks to the web at the areas of contact. It is believed that a combination of these possibilities may operate to produce transfer-developement according to the present invention.

The mechanism of the transfer-development embodiment shown in FIG. 5 is believed to be the same as that relating to the embodiment shown in FIG. 4 but With an additional phenomenon. Image receiving web 34 may be desirably at least mildly conductive so as to present an equipotential surface adjacent to the charged plate. With exposure to the pattern of light and shadow, the charge applied to the plate is partially and selectively balanced by the migration of charge of opposite polarity through photoconductive layer to a substantially closer proximity to the developer layer. The same migration of charge through the photoconductor does not occur in the dark areas, however, in the event of conductivity of image receiving web 34, lateral migration of charge may occur to maintain an equipotential surface, resulting in a varying electrical field between the plate and the equipotential surface. Accordingly, image receiving web 34 is belived to receive a concentration of charge in the dark areas of such polarity to promote transfer of toner particles onto the web.

Several different materials have been tried and found suitable for use as the image receiving web in the production of copies. according to the instant invention. Electrically conductive as well as electrically nonconductive materials including various bond papers, baryta-coated papers, plastic films such as Mylar film and Saran film, steel and brass foil, aluminum and paper M-ultilith masters as well as very thin aluminum foil have been used successfully. Bond paper, because of its absorbent characteristics, was tfOllIld to produce prints with considerable background. It is believed that the coarse surface of bond paper is a detrimental factor because of a' tendency to entrap toner particles in non-image areas. Higher quality, relatively background-free prints can be made on smoother, less absorbent materials such as, for example, baryta-coated paper.

Developer compositions suitable for use in the present invention comprise dispersions of toner particles formed in a liquid carrier for example by stirring or other methods well known to those skilled in the art. It is understood that the toner particles maybe liquids, solids or organosols capable of being dispersed in the carrier. Thus, suitable dispersions, suspensions, and emulsions and other colloidal systems are within the scope of this invention.

The carrier liquid is preferably a highly insulating material with a volume resistivity of at least about 10 ohmcentimeters. Suitable liquids include hydrocarbons such as benzene, xylene, hexane, naphtha, cyclohexane, etc.; halogenated hydrocarbons such as carbon tetrachloride, trichloroethylene, chloroform, Freons (a trade name of E. I. du Pont de Nemours & Co. for various fluorinated compounds), Genetrons (a trade name of Allied Chemical Co. for various fluorinated compounds), etc.; miscellaneous liquids such as silicone liquid, turpentine, etc. A particularly useful material of refined petroleum hydrocarbon class is Sohio Odorless Solvent (SOS) sold by the Standard Oil Company of Ohio.

If desired, the carrier may comprise a material that is a solid at room temperature such as Bioloid parafiin wax. In such applications, an additional step is required in practicing the present invention to temporarily liquify the developer layer. FIG. 5 illustrates liquificat-ion of the layer of developer 11 through the application of heat from adjacent heating element 37 or the like. Alternative liq-uification methods may be employed, depending upon the composition of the carrier. For-instance, the devolper may be exposed to a vapor solvent for the carrier to accomplish the same functional result.

It will also be apparent to those skilled in the art that an embodiment within the scope of the present invention may comprise employing a developer-coated image receiving web instead of a developer-coated plate. An image receiving web coated with developer having a paraffin wax carrier may be found suitable for this embodiment. The devolper layer may be liquified by, for instance, the method shown in FIG. .6, and then transferdevelopment carried out either by rolling as shown in FIG. 4 or simultaneously with exposure as shown in FIG. 5. Alternatively, the liquification step may be combined with transfer-development by, for instance, using a heated roller or positioning heating element 29 of FIG. 6 adjacent to the combination of elements shown in FIG. 5.

. Liquid toner particles must be substantially immiscible with the carrier liquid and capable of being dispersed in the form of very small droplets. An almost limitless number of suitable materials exist which may be used in conjunction with the present invention. One suitable class of materials comprises aqueous inks such as ordinary commercial fountain pen inks. Another particularly useful class of materials includes glycols, preferably including soluble dyes in order to render the image visable.

Particularly suitable examples include ethylene glycol containing about 5% of crystal violet or malachite green dye.

Suitable solid toner particles include pigments, dyes, resins, metals, and materials such as charcoal and graphite and similar material, having a fine particle size and being insoluble in the carrier liquid. In general, thefiner the powder, the better the grain in the devoloped image. It is prefered that the average particle size be no larger than about 20 microns and for high quality work it is prefered that the average particle size be no more than about 5 microns.

As mentioned above, the toner image is typically rendered permanent by a separate fixing step such as heat fusing or the like. If desired, the separate fixing step may be obviated by an appropriate selection of developer components. For instance, self-fixing characteristics may be imparted to the developer by using toner particles comprising resins which are binders or filmforming. A composition suitable for this purpose comprises an organosol in which a solution of a resin binder in a polar solvent is dispersed in a non-polar carrier.

Another suitable composition comprises a suspension of finely divided resin dyestuff in molten paraffin wax. solidification of the wax upon cooling to room temperature serves to fix the image to the image receiving web.

Limitation by the specific embodiments of the invention as set forth in this application is not intended. Rat-her, it is intended that the claims apply broadly within the spirit and scope of this invention.

What is claimed is:

1. The method of xerographic copy reproduction comprising sequentially:

(a) applying to the photoconductive insulating layer of a xerographic plate a layer of developer comprising particulate toner suspended in a non-conductiveliquid carrier and electrostatically charging the xerographic plate; and

(b) exposing said photoconductive insulating layer to a'pattern of light and shadow and placing the surface of an image receiving web in contact with said layer of developer to selectively deposit toner particles on said image receiving web.

2. The method according to claim 1 in the sequence in which said image receiving web is placed in contact with said layer of developer prior to exposing said plate to said pattern of light and shadow.

3. The method of xerographic copy reproduction comprising sequentially:

(a) applying to the photoconductive insulating layer of a xerographic plate a layer of developer comprising particulate toner suspended in a non-conductive liquid carrier and electrostatically charging the xerographic plate; and

(b) exposing said plate to a pattern of light and shadow and bringing one surface of an image receiving web into contact with said layer of developer by 6 rolling a cylindrical roller across the opposite surface of said web to selectively deposit toner particles on said image receiving Web.

4. The method according to claim 3 wherein said 8. The method according to claim 3 wherein said particulate toner comprises a solution of resin in a solvent therefor and said liquid carier comprises a non-polar organic liquid.

9. The method according to claim 3 wherein said toner image is fixed to said image receiving web by heat fusing.

10. The method according to claim 3 wherein said particulate toner comprises a solution of resin in a solvent therefor and including the additional step of fixing said toner particles deposited on said image receiving web by eliminating the solvent therefrom.

' 11. The method of xerographic copy reproduction comprising sequentially:

(a) applying to the photoconductive insulating layer of a xerographic plate a layer of developer comprising particulate toner suspended in a non-conductive liquid carrier and electrostatically charging the xerographic plate; I

(b) exposing said plate to a pattern of light and shadow to cause the developer in the dark areas to be raised with respect to the developer in the light areas;

(c) bringing the surface of an image receiving web at least into contact with the selectively raised portions of the developer to eifect selective transfer of particulate toner to said web; and (d) separaing said web from said developer References Citedv by the Examiner UNITED STATES PATENTS 2,847,305 8/1958 Walkup 96-1 2,868,642 1/ 1959 Hayford et al 96-1 2,899,335 8/ 1959 Staughan 96-1 2,924,519 2/ 1960 Bertelsen 96-1 2,940,847 6/ 1960 Kaprelian 117-17.5 3,013,890 12/ 1961 Bixby 96-1 3,053,962 9/ 1962 Cerasani et a1 96-1 3,071,645 1/ 1963 McNaney 117-175 3,120,446 2/ 1964 Hunter 117-175 NORMAN G. TORCHI'N, Primary Examiner. A. LIBERMAN, D. P. PRICE, Assistant Examiners. 

1. THE METHOD OF EXEROGRAPHIC COPY REPRODUCTION COMPRISING SEQUENTIALLY: (A) APPLYING TO THE PHOTOCONDUCTIVE INSULATING LAYER OF A XEROGRAPHIC PLATE A LAYER OF DEVELOPER COMPRISING PARTICULATE TONER SUSPENDED IN A NON-CONDUCTIVE LIQUID CARRIER AND ELECTROSTATICALLY CHARGING THE XEROGRAPHIC PLATE; AND (B) EXPOSING SAID PHTOCONDUCTIVE INSULATING LAYER TO A PATTERN OF LIGHT AND SHADOW AND PLACING THE SURFACE OF AN IAMGE RECEIVING WEB IN CONTACT WITH SAID LAYER OF DEVELOPER TO SELECTIVELY DEPOSIT TONER PARTICLES ON SAID IMAGE RECEIVING WEB. 